Fan Xiu Lu

Improvement of Adhesion of Cubic Boron Nitride Films: Effect of Interlayer and Deposition Parameters

Diamond and B4C coatings were used as an interlayer for the growth of cubic boron nitride thin films on c-silicon. By employing a B-C-N gradient layer on top of the B4C interlayer, improved adhesion occured between BN and B4C. A multi-step process after the nucleation of c-BN was found very helpful for improving the adhesion of c-BN on silicon with interlayers. Residual stress of c-BN thin film was significantly decreased by using a new post-deposition annealing treatment.

Effect of Growth Parameters on Single Crystal Diamond Deposition by DC Arc Plasma Jet CVD

Homoepitaxial diamond layers were grown on commercial 3.5 x 3.5 x 1.2 mm3 HPHT synthetic type Ib (100) single crystal diamond plates using a DC Arc Plasma Jet CVD operating at gas recycling mode. The effects of substrate temperature and CH4/H2 ratio on the surface morphology, the growth rate and the quality of the synthesized diamond have been studied using optical microscopy and Raman spectroscopy. With no intentional nitrogen added, the growth rate up to 12.3µm/h has been obtained in the single crystal diamond sample deposited at 1000 °C with CH4/H2=0.625%, exhibiting relatively smooth surface morphology without any growth hillocks nor non-epitaxial crystallites, and presenting the typical feature of the epitaxial step-flow growth. The full width at half maximum (FWHM) of the Raman spectra was 2.08 cm-1, which was close to that of the natural type IIa single crystal diamond.

High Current Extended DC Arc Plasma CVD for Mass Production of Diamond Film Coated Hard Metal Cutting Tools

Diamond film coated hard metal cutting tools are indispensible for high efficiency machining of materials which are difficult to cut by ordinary tools, and are successfully used in the dry cutting of high silicon content Al-Si cast alloys, graphite, carbon reinforced composite (CRFC) and metal matrix composite (MMC) , ceramics, and many other materials. In the present presentation, a novel process of High Current Extended DC Arc (HCEDCA) plasma CVD for mass production of diamond film coated hard metal cutting tools is presented. Besides, a novel process for the pretreatment of the hard metal cutting tool substrate, which involves the idea of “surface engineering” consisting of boronizing and alkaline and acidic etching is also discussed, by which the adhesion of the diamond film coating to the hard metal substrate can be greatly enhanced. Highly adherent and uniform diamond film coatings are successfully obtained. Diamond film coated WC-6wt%Co indexable tool bits, drills, endmill samples have been produced and been shown having excellent cutting performance by field cutting tests in dry cutting of Al-12%Si cast alloy and Al-15% SiC MMC materials.

A Review on Mechanical Properties of Freestanding Diamond Films

As an emerging brand new type of engineering material for a variety of important high technology applications, the deep understanding of the mechanical behavior of freestanding diamond films has become an emergent task of vital importance. Of the many deposition methods dc arc plasma jet has been regarded as the most promising technique for large area high quality and low cost production of freestanding diamond films. In the present paper, recent progress in mechanical properties of freestanding diamond films mainly by high power dc arc plasma jet with rotating arc root and gas recycling is reviewed. Testing methods for fracture strength and fracture toughness are discussed. Experimental data are presented and compared to that by MWCVD. Dc arcjet diamond films start to oxidize at about 700°C, however, oxidation up to 800°C for 10 min does not affect the fracture strength. Fracture mechanism is discussed. The strange mechanical behavior of freestanding diamond films is explained. It is surprising that CVD diamond film is such a kind of material which is strong, but full of different size of defects. It is hoped that the present paper will be helpful for those who wish to understand and use this brand new type of engineering material.

Low roughness thin diamond films produced at moderate temperatures using microwave plasma assisted chemical vapour deposition process

Abstract Diamond coatings on pure titanium substrates are of interest for tribological and biomedical implants; however, due to different thermal expansion coefficients of the two materials, the complex nature of the interlayer formed during diamond deposition, and the difficulty in achieving very high nucleation density, it is hard to deposit adherent thin diamond layers on titanium. The aim of this research work is to successfully produce smooth and well adherent diamond coatings on a pure Ti substrate using the microwave plasma chemical vapour deposition (MWPCVD) method. The effects of applying two kinds of gas mixtures (H2/CH4 and Ar/H2/CH4) during diamond deposition on the pure Ti substrate are studied. The morphology, surface roughness, diamond crystal orientation and purity are obtained by characterising the sample with field emission electron microscopy (FESEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy respectively.

Residual Stress Analysis of Microwave Plasma CVD Diamond Films

The residual stress and crystallographic texture of diamond films were investigated in the present work. The diamond films were synthesized on (100) silicon wafer by Microwave Plasma Chemical Vapor deposition (MPCVD). Then the residual stresses of the films were measured by X-ray diffractometer equipped with the two-dimensional detector. The residual stresses can be classified into two categories, i.e., the intrinsic stresses and the thermal stresses. It was shown that the thermal stresses were compressive in the temperature range studied and the intrinsic stresses were tensile. The crystallographic textures of the films were measured by X-ray diffractometer with the method of pole figure and orientation distribution function (ODF). The experimental results suggest that the crystallographic textures of the films depend upon the deposition temperature and methane flow rates, and the components and intensity of crystallographic textures have effect on the residual stresses in diamond films to a certain extent.

Microstructure and Texture in Free-Standing CVD Diamond Films

Free-standing CVD diamond films were prepared under the substrate temperature in the range of 850-1050oC. Macro- and micro-textures of the films were investigated based on the SEM observation as well as on the ODF and EBSD analysis. It was found that certain growth selection process appeared during diamond deposition which, however, did not lead to a strong film texture. It is indicated that strong fluctuation of growth ratio V<100>/V<111> and frequent growth twinning during film deposition resulted in randomization effect of grain orientations, which can be transformed by adjusting the parameters of film preparation.

Application of High Power DC Arc Plasma for Mass Production of High Quality Freestanding Diamond Films and Diamond Film Coated Cutting Tools

As quasi-thermodynamic equilibrium plasma, DC Arc Plasma has the advantage of very high gas temperature and thus the very high degree of activation of the precursors for diamond film deposition. The present paper reviews the progresses in the R&D of the novel high power dc arc plasma jet CVD system with rotating arc and operated at gas recycling mode for large area high quality diamond film deposition, developed at the University of Science and Technology Beijing (USTB) in the mid 1990s of the 20th century. Thanks to the continuous efforts made in the technological improvement in the past 15 years, considerable progresses have been achieved in the commercialization of this high power dc arcjet CVD system, which is now capable of mass production of large area high quality freestanding diamond films for optical, thermal, and mechanical (tool) applications. The present status in the commercialization and the property level of the resultant diamond films in optical, thermal, mechanical, dielectric, oxidation resistance, sand erosion resistance, and laser damage threshold etc. are presented. Based on the same high power dc arcjet technology, a novel high current extended dc arc plasma (HCEDCA) CVD system has been developed which successfully changed the diamond film deposition mode from 2D planar deposition in to 3D deposition (as confined by two hollow (virtue) columns). It is demonstrated to be advantageous for mass production of diamond thin film coated WC-Co cutting tools. Recent results in the R&D of thin diamond film coated WC-Co drills and end mills, and the results in field tests are discussed.

Effect of External Magnetic Field on the Flow and Heat Transfer in DC Arc Plasma Torch

A multiple fields’ coupled model of new magnetic controlled DC plasma torch, which was used for CVD diamond film, was presented. In this model, the effects of electric field and magnetic field on the flow field and temperature field were taken into account, and the fluid dynamics equations were modified by the addition of some source terms relating to electromagnetic field, such as Lorentz force, joule heating, and radiative cooling. Conversely, the generalized ohm’s law was used to solve the current density, which reflected the effects of flow field and temperature field on the electric field and magnetic field. In addition, the rest Maxwell’s equations and external solenoid magnetic field equation were also modeled. In order to know the effect of external magnetic field on the torch, the current intensity of external solenoid was chosen to simulate its influence on the flow and heat transfer in the torch. Results show that external magnetic field plays a part in stirring the plasma, which is advantageous for the preparation of diamond film. The larger the external solenoid current intensity is, the better the uniformity of the temperature and velocity of plasma is.

Finite-Element Analysis of Thermal Stresses in Diamond Film Deposition on Mo Substrate with Ti Interlayer

The effects of suppressing film cracking during large area freestanding diamond films deposition on Mo substrates with Ti interlayer were explored based on the finite-element code ANSYS. The analysis results were verified by experimental results. It showed that during large area freestanding diamond films deposition, Mo substrates with Ti interlayer have good effects on thermal stress relaxation and reduce the probability of crack initiation. High comprehensive quality diamond films were obtained at 950°C.